Cellular mobile telephone systems rely on the reuse of radio frequencies in different cells or radio coverage areas. When a mobile terminal moves from communication with a base station in one cell (original base station) to another it is necessary to “handover” the mobile terminal to the base station (target base station) of the new cell. Handovers may be passive or active, i.e. a handover may be necessary when there is no active communication and the mobile terminal is “camped” on the current cell and must be transferred to the new cell (passive handover) or when there is an active communication which must be transferred from the current to the target base station (active handover). In order to avoid loss of data during an active handover it is preferred if the handover is quick. In order to improve the speed of handovers both “seamless” and “soft” handovers have been proposed. A seamless handover is a “hard” handover (new connection to the target base station is made at the time of loss of the old connection) in which the network connections up to the target base station are reserved and synchronised so that when the actual handover takes place there is the minimum of interruption between breaking the communication with the old base station and connecting through to the new base station. In a soft handover the mobile terminal communicates with both the old base station and the target base station at the same time and the network may decide, based on certain communication qualities or other criteria, when the signals arriving via the target base station are acceptable and the link to the original base station may be broken.
Hard handover methods may suffer from suddenly varying signal strength. The deterioration of the signal quality of communications with the original base station may be masked by shadowing and widely varying fading on the current communication channel so that the reduction in signal quality which triggers the handover may occur suddenly and catastrophically (for example, the “corner effect”). In this situation communication with the original base station is lost suddenly and a handover procedure which relies on a handover request being sent to the original base station may fail. Schemes have been proposed to deal with this situation, e.g. on loss of communication with the original base station the mobile terminal attempts a forward handover, i.e. it tries to make contact with a target base station and the network attempts to set up the new route through the network from the target base station. Setting up the new network path takes some time, particularly as the network has not been prepared in any way for this emergency operation, so that it is almost impossible with this method not to loose some data. Very often the complete communication is lost.
An alternative is to allow communication with more than one base station at the same time so that sudden loss of communication with one base station has no affect on the current communication. A method and a system for providing a communication with the mobile terminal through more than one base station during the handover process are disclosed in U.S. Pat. No. 5,625,876. Using this system, a communication between the mobile terminal and the end user is not interrupted by a handoff from the original base station to a target base station. This type of handover may be considered as a “soft” handover in that communication with the target base station is established before communication with the original base station is terminated. When the mobile terminal is in communication with two base stations, a single signal for the end user may be created from the signals from each base station by a cellular or personal communication system controller.
In systems in which a mobile terminal may communicate with several base stations at the same time, e.g. CDMA systems, mobile terminal assisted handoff may operate based on the signal strength of beacon or pilot signals from several sets of base stations as measured by the mobile terminal. An Active Set is the set of base stations through which active communication is established. The Neighbour Set is a set of base stations surrounding an active base station comprising base stations that have a high probability of having a pilot or beacon signal strength of sufficient level to support communication of adequate quality. The Candidate Set is a set of base stations having a pilot or beacon signal strength of sufficient level to establish communication.
When communications are initially established, a mobile terminal communicates through a first base station and the Active Set contains only the first base station. The mobile terminal monitors the pilot or beacon signal strength of the base stations surrounding at and each of these is allocated to the Active Set, the Candidate Set, or the Neighbour Set. When a pilot or beacon signal of a base station in the Neighbour Set exceeds a predetermined threshold level, the base station is added to the Candidate Set and removed from the Neighbour Set of the mobile terminal. The mobile terminal communicates a message to the original base station identifying the new base station. A cellular or personal communication system controller decides whether to establish communication between the new base station and the mobile terminal. Should the cellular or personal communication system controller decide to do so, the cellular or personal communication system controller sends a message to the new base station with identifying information about the mobile terminal and a command to establish communications therewith. A message is also transmitted to the mobile terminal through the original base station. The message identifies a new Active Set that includes the original and the new base stations. The mobile terminal searches for the new base station transmitted information signal and communication is established with the new base station without termination of communication through the original base station. This process can continue with additional base stations.
When the mobile terminal is communicating through multiple base stations, it continues to monitor the signal strength of the base stations of the Active Set, the Candidate Set, and the Neighbour Set. Should the signal strength corresponding to a base station of the Active Set drop below a predetermined period of time, the mobile terminal generates and transmits a message to report the event. The cellular or personal communication system controller receives this message through at least one of the base stations with which the mobile terminal is communicating. The cellular or personal communication system controller may decide to terminate communications through the base station having a weak pilot or beacon signal strength.
The cellular or personal communication system controller upon deciding to terminate communications through a base station generates a message identifying a new Active Set of base stations. The base station through which communication is established sends a message to the mobile terminal. The cellular or personal communication system controller also communicates information to the relevant base station to terminate communications with the mobile terminal. The mobile terminal communications are thus routed only through base stations identified in the new Active Set. In a cellular or personal communication telephone system, maximising the capacity of the system in terms of the number of simultaneous telephone calls that can be handled is also extremely important. If the mobile terminal is communicating simultaneously with several base stations, for example, when the mobile terminal is close to the boundaries of several different cells, additional radio and network resources are occupied which reduces the total capacity of the system. Hence, it is advantageous to limit the number of simultaneous communications of the same data using up radio capacity. Further, a high bit-rate, high priority mobile terminal which approaches a cell boundary may be operating at high power due to poor communication channels. This high power signal may seriously increase the noise level in the target cell. Generally, fast power control of the mobile terminal from the target base station will only be effective after the soft handover is complete. During this delay the users in the target cell may receive poor communication quality and/or loss of their calls.
As mobile cellular telephone systems have become more sophisticated, the handover requirements have also become more complex. In particular, there is an increase in the types of communications supported by mobile telecommunication systems and the requirements of these communications may be widely different. Important parameters of any communication may be its urgency, i.e. how quickly must the handover take place and how much delay or interruption is acceptable, its bandwidth, its minimum or preferred signal quality, its data rate, its priority, i.e. is the communication a premium service which may take priority over less important messages. Conventionally, certain types of data are considered to require a very quick handover with little loss or delay of the communication, e.g. telephone conversations. The increasing use of Internet telephones indicates, however, that delays in voice communications may be acceptable provided there is a significant cost saving. In addition, there is an increasing use of voicemail. Some telephone users only use their fixed telephones for outgoing calls and switch all ingoing calls to voicemail leaving them undisturbed until they wish to create a communication. With voicemail it is only necessary that the voice information is recorded accurately, exactly when it arrives is not of such a great importance. These changes in voice communications mean that different levels of service may be provided by the mobile communication network operator, e.g. a high price voice communication with high urgency and priority and a low price voice communication in which delays in duplex communications are possible and have to be accepted. On the other hand, certain types of data transmissions suffer greatly if there is an interruption during handover. Also, the bandwidth for different types of voice or data transmissions may vary widely, e.g. video, computer file downloads, voice or fax. Thus, there are several different qualities of service (QoS) which may be required for any particular transmission. During a handover the required QoS of the current transmission may be one of the parameters which influences the handover decision, i.e. can the new path support the required QoS. Checking the QoS for the new path requires knowledge about the properties of the new route from the target base station through the network to the end user to decide whether or not it meets the minimum QoS or the present QoS of the current transmission. This increase in sophistication of the handover decision increases its complexity and the time taken for its successful completion. For example, if one of the QoS requirements of an existing transmission is its priority i.e. it has a higher priority than other transmissions in the old cell or new cell it may be necessary to terminate lower priority communications in the target cell or to arrange for them to be handed over to another cell before the high priority communication can itself be handed over.
Summarising the above, there is a need for a very fast handover which can accommodate QoS requirements such as priority, urgency, bandwidth or cell or packet delay and which is easy to implement.